1996 Overheating Issues

1996 Monte Carlo V6-201 3.4L DOHC 103,000 Miles

I have taken the car into the shop twice for overheating problems: The first time was for a coolant leak (water pump), the shop flushed / filled coolant, replaced water pump and thermostat. The second time, the shop replaced the coolant temperature sensor.

The car runs fine and does not overheat, if I run A/C constantly. Both electric cooling fans run with A/C ON. This morning I tried driving with A/C OFF to save gas. I drove about 8 miles and the temperature gauge needle went into the red, and the overtemp warning light illuminated. I pulled over, opened the hood, and the electric fans are not running. I did not see any steam or coolant leaks, and overflow reservoir is at the "Hot" level. I shut the engine off, and waited a few minutes, restart with A/C ON. Temperature gauge drops quickly...

It's really weird, because I can start the car and let it idle in my driveway. The driver's side cooling fan comes on with the A/C off, and temp gauge needle in the middle.

I don't see any signs of coolant in the oil, or any coolant loss.

Can you help please?

Welcome aboard!

The PCM should shut off the fans at about 38-41 MPH, since they do little good at that speed anyway. The outputs which operate both of the fan relays for the A/C pressure switch is the same one which operates the fans based on the CTS data. AFAIK, teh PCM should turn the fans back on below about 35 MPH. There may be a duty cycle programmed at certain temperatures and/or speeds, but that should not be a factor once the vehicle has stopped. Typically, the primary fan will turn on at about 223°F, and the secondary fan will turn on about 235°F, but those exact numbers vary a bit between years and models. The point is that the PCM will operate the fans, so the fan relays and PCM outputs are not suspect. The only question is the accuracy of the data sent to the PCM by the CTS.

The sending unit which operates the dash gauge AND the warning lamp, however, is a different device entirely. The shop should have checked the CTS data and compared it to the dash gauge to help determine whether either one (or both) was at fault. Since there is no sign of boiling over, venting coolant from overpressure, or other obvious overheating, I would suspect the coolant temperature is remaining below 245°F. The red line/warning point of the gauge (and lamp) should be somewhere around 260°.

Thanks for the nice welcome, and for your help. :) I did not know the Temp Gauge Sensor also controls the warning lamp -- very informative thanks for pointing that out. I will take it back to the shop, and request they compare the Coolant Temp Sensor data with the Temp Gauge.

The temperature gauge sending unit operates the gauge directly, and also operates the warning lamp through a control module. Since the gauge reads full hot and the warning lamp does operate at that point, I would suspect that part of the circuit is operating normally. It's relatively easy to compare the CTS and sensing units with just an ohmmeter, or with a scanner and an ohmmeter. Either one should present a resistance close to the table at a given temperature:

http://72.19.213.157/files/CTSMAT.gif

I'm not sure if my camaro 96 has same engine as your 96 monte carlo, but I'm having the same problem. Early this year when I noticed it overheating, I found sludge in my radiator. I've flushed it 4 times already. Changed thermostat and replaced water pump when it started to leak. RIght now I'm soaking it SUper Flush system and I'm going to drain it this weekend and see if it still overheats when A/C is not turn on.

I am having the same problems.

I replaced a leaking water pump and flused my coolant and it seems all hell has broken loose.

I stated to run hot 3/4 to red line while accelerating in town and driving on the higway. I would put the car in nuetral while driving and rev up to 5,000 RPM and the temp gauge would immediatly go to 1/4 or 1/2. I would shut off the car for a minute and then the start up to find the the temp went down to 1/4 or 1/2 way. I started to open the bleeder screw on the thermostat housing (while the engine was off and hot) and notice that only air was coming out. I was advised that the intake manifold gasket was bad. the intake air was escaping into the coolant passage due to a bad seal around the coolant passage (i have pictures). Air was getting trapped around the temp sensor and not sensing the temperature of coolant. It was sensing air temp; there not made to sense air temp. So, my fan was never turning on. when i removed the air it ran at normal operating temperature (good fan Cycles) at idle speed in the driveway until i put a good amount of load and we would start all over.

After replacing the head gasket; Cylider heads (because of valve guide sliping), intake manifold gasket, temp sensor, radiator hoses, thermostat.
I have reversed the problem. Know the engine runs hot after 10 min from sitting over night on the road the fan will never turn on. I will purge air out of it and it will run and normal operating temp the rest of the day, fan going on and off when needed. After sitting over night the problem will repeat itself. I have been doing this for a week and I figured that thier is just air in system from filling after fininshing the repairs.

I am not having any coolant looses either

Is any one having the same problem.

So this weekend I did drain the flush system from the radiator of my camaro 96 after soaking it for one week due to sludge build up. I filled it with green coolant NOT orange. My manual says you can use either. It's a lot better better and fan is cycling like the book says. I live in FLorida and the it's always warmer and hot. I wanted to the radiator fans to kick running lower than the factory setting temperature. By experimenting with the coolant sensor sensor resistance responsible for the triggerring of the fan and the gauge reading, I connected a 1 K ohm resistor in parallel with the sensor by splicing the yellow and the black wire connected to the sensor to lower the starting resistance. This sensor reacts, as the engine temp goes higher the sensor resistance goes lower. So now, I have a lower starting resistance, so when the engine goes gets hotter the resistance goes lower faster that the original fatory setting and computer senses it and turns the radiator fans earlier. I tested it by drivng around without the A/C on and on my drive way. It work really ggod and I'm happy that the engine runs much cooler. The fans acutally works gentlier as it turns off and On because the effect of reistance change is slower when I connected the 1 K ohm resistor with the variable Temperature sensor resistor. I found out that the fans speed is dependent on the engine temperature, the hotter it is, the fans goes faster. As I put my hand infront of the radiator fan, I can feel the fan goes slower and slower until it auto shuts off as the engine is cooling off. Then turns on slowly again as the engine gets hotter.
I have a 3.8 engine 6 cyl 96 camaro, but the circuit might same as your monte carlo